CRITICAL-LEVEL RESONANCE IN 3-DIMENSIONAL FLOW PAST ISOLATED MOUNTAINS

A set of numerical simulations with a three-dimensional nonhydrostatic model is used to investigate the behavior of the atmospheric flow pas t idealized isolated mountains in the presence of an environmental cri tical level aloft. The study addresses the problem of three-dimensiona l effects on the generation of high-drag Bow regimes as a function of the critical level height, concluding that those effects can lead to s ignificant changes in the preferred heights for resonance. The results are compared with theories that have been proposed to explain the hig h-drag states in two-dimensional flow with critical levels and it is f ound that, while some of their predictions hold in three dimensions, t here is not only an overall change in the amplitude of the effects but also an essential modification of the preferred locations of the crit ical level height lending to resonance. Whereas two-dimensional studie s have shown a vertical spacing between resonant critical level height s very close to one hydrostatic wavelength, the present results show a clear half-wavelength periodicity, as in classic linear resonance. Bo th the latter result and the much reduced ''resonance shift'' observed in the present study seem to indicate that the two-dimensional hydrau lic theory cannot be applied to circular mountains without significant modification. Some other significant differences between two- and thr ee-dimensional results are shown and related to both the linear and no nlinear behavior of the three-dimensional unsheared flow. For comparis on, some three-dimensional simulations of Bow past infinite ridges are also presented and they are found to be very similar to previous two- dimensional studies.